Non-myeloablative conditioning with allogeneic hematopoietic cell transplantation for the treatment of high-risk acute lymphoblastic leukemia

Abstract

Background Allogeneic hematopoietic cell transplantation is a potentially curative treatment for patients with acute lymphoblastic leukemia. However, the majority of older adults with acute lymphoblastic leukemia are not candidates for myeloablative conditioning regimens. A non-myeloablative preparative regimen is a reasonable treatment option for this group. We sought to determine the outcome of non-myeloablative conditioning and allogeneic transplantation in patients with high-risk acute lymphoblastic leukemia.Design and Methods Fifty-one patients (median age 56 years) underwent allogeneic hematopoietic cell transplantation from sibling or unrelated donors after fludarabine and 2 Gray total body irradiation. Twenty-five patients had Philadelphia chromosome-positive acute lymphoblastic leukemia. Eighteen of these patients received post-grafting imatinib.Results With a median follow-up of 43 months, the 3-year overall survival was 34%. The 3-year relapse/progression and non-relapse mortality rates were 40% and 28%, respectively. The cumulative incidences of grades II and III-IV acute graft-versus-host disease were 53% and 6%, respectively. The cumulative incidence of chronic graft-versus-host disease was 44%. Hematopoietic cell transplantation in first complete remission and post-grafting imatinib were associated with improved survival (P=0.005 and P=0.03, respectively). Three-year overall survival rates for patients with Philadelphia-negative acute lymphoblastic leukemia in first remission and beyond first remission were 52% and 8%, respectively. For patients with Philadelphia chromosome-positive acute lymphoblastic leukemia in first remission who received post-grafting imatinib, the 3-year overall survival rate was 62%; for the subgroup without evidence of minimal residual disease at transplantation, the overall survival was 73%.Conclusions For patients with high-risk acute lymphoblastic leukemia in first complete remission, non-myeloablative conditioning and allogeneic hematopoietic cell transplantation, with post-grafting imatinib for Philadelphia chromosome-positive disease, can result in favorable long-term survival. (Clinicaltrials.gov identifier: NCT0036738)

Introduction

Allogeneic hematopoietic cell transplantation (HCT) with a myeloablative conditioning regimen is an established potentially curative treatment for patients with high-risk acute lymphoblastic leukemia (ALL).1,2 However, many older adults with ALL are not candidates for high-dose conditioning and HCT.3,4 The use of non-myeloablative conditioning with fludarabine and low-dose total body irradiation can substantially decrease the toxicity of the preparative regimen and extends the possible use of allogeneic HCT to older or medically infirm patients.5,6 This approach relies primarily on potent graft-versus-leukemia effects to prevent relapse of the disease. Other reduced intensity regimens have been reported by investigators for the treatment of patients with ALL.3,7–9 However, relapse has remained a major problem following reduced intensity conditioning regimens.

In recent years, imatinib mesylate, and subsequently the newer tyrosine kinase inhibitors, dasatinib and nilotinib, combined with chemotherapy were found to be very effective for inducing disease remission in patients with Philadelphia chromosome-positive (Ph+) ALL.10–13 Imatinib therapy after allogeneic HCT was well tolerated and improved relapse-free survival following myeloablative conditioning compared to that in historical controls not given imatinib therapy after HCT.14 A recent study showed that patients with Ph+ ALL who received induction chemotherapy with imatinib followed by myeloablative conditioning and allogeneic HCT for Ph+ ALL in first complete remission had a better overall survival compared to patients who did not undergo HCT.15

Here we report on the multicenter experience with allogeneic HCT following non-myeloablative conditioning with fludarabine and 2 Gray (Gy) total body irradiation for patients with high-risk ALL. We identify risk factors for disease relapse and mortality. We also describe the causes of non-relapse mortality and the toxicity and efficacy of post-HCT imatinib for patients with Ph+ ALL.

Design and Methods

Eligibility

This analysis includes 51 consecutive patients with ALL who were prospectively enrolled and received non-myeloablative conditioning followed by allogeneic HCT on sequential multi-institutional protocols between February 1, 2000 and July 30, 2009. The protocols were registered as National Cancer Institute clinical trials. Patients treated with post-grafting imatinib were registered in NCT00036738. Other patients were enrolled in sequential protocols specific for donor type and with minor variations in planned duration of post-grafting immunosuppression. Patients were treated at six centers with the Fred Hutchinson Cancer Research Center (FHCRC) in Seattle (WA, USA) acting as the coordinating center. All patients signed informed consent forms approved by the local institutional review boards.

Patients with related or unrelated donors were eligible for non-myeloablative conditioning if they were older than 55 or 50 years, respectively. Younger patients were eligible if they had high-risk ALL and co-morbid conditions that excluded them from myeloablative conditioning or if they had disease relapse after a preceding myeloablative HCT. Adult high-risk ALL was defined as beyond first complete remission, or first complete remission and at least one of the following: (i) age greater than 35 years, (ii) white blood cell count greater than 30×10/L at diagnosis for B-cell ALL or greater than 100×10/L at diagnosis for T-cell ALL, or (iii) Ph+ ALL with t(9;22).2 Pediatric high-risk ALL was defined as beyond first complete remission, or first complete remission and the addition of one of the following: (i) failure to achieve complete remission after the induction phase; (ii) t(9;22) or t(4;11) clonal abnormalities; and (iii) poor response to prednisone in T-cell ALL with a white blood cell count greater than 100×10/L.16

Pre-transplant characteristics

Patients referred for HCT in first complete remission had had a median of three cycles (range, 3–4) of various standard induction/intensification chemotherapy regimens.17–19 Pre-transplant disease status was assessed within the 21 days prior to HCT. Complete remission was defined according to standard morphological criteria as outlined by the International Working Group.20 For patients in complete remission, minimal residual disease was assessed by multiparametric flow cytometry (minimum four-color, cut-off level to establish minimal residual disease positivity ≥ 0.01%), karyotype analysis (G-banding) and fluorescence in situ hybridization (FISH). Quantitative reverse transcriptase–polymerase chain reaction (RT-PCR) of p210 and p190 BCR/ABL mRNA was not part of the work-up for minimal residual disease in all of the patients with Ph+ ALL; thus, PCR results were not included in assessment of minimal residual disease. All patients and donors had high resolution HLA-allele level typing performed for ten HLA alleles (HLA-A, B, C, DRB1 and DQB1). Patients received grafts from the following donors: HLA identical siblings (n=9), 10/10 HLA-allele matched unrelated donors (n=31), single HLA allele mismatched unrelated donors (n=6) and single HLA-antigen mismatched unrelated donors (n=5). Pre-transplant comorbidities were assessed retrospectively, using the HCT comorbidity index (HCT-CI).21

Conditioning regimen and treatment plan

The conditioning regimen consisted of fludarabine (30 mg/m/day on days -4 through -2 before HCT and 2 Gy total body irradiation given at a dose of 0.07 to 0.1 Gy/min from linear accelerator sources on day 0.6 Patients received unmanipulated granulocyte colony-stimulating factor-mobilized peripheral blood stem cells shortly after total body irradiation. One patient received an unrelated bone marrow graft instead of peripheral blood stem cells. Post-grafting immunosuppression consisted of combined mycophenolate mofetil and a calcineurin inhibitor, cyclosporine or tacrolimus, as previously described.5,22–25

All patients received intrathecal methotrexate (12 mg/dose or 6 mg/dose if given via an Ommaya reservoir) for central nervous system prophylaxis, two doses pre-transplant (once per week) and six doses post-transplant (starting on day +30, once every 2 weeks). All men received 16 Gy of testicular irradiation in eight fractions during conditioning. All patients with a history of central nervous system disease, three with Ph– and four with Ph+ ALL, received cranio-spinal irradiation as part of the conditioning regimen.

The mean infused dose of peripheral blood stem cells was 8.5×10 CD34 cells/kg body weight (range, 0.9×10–24.4×10). Engraftment and donor chimerism were measured by variable number tandem repeat of microsatellite markers at days 28, 56 and 84 after HCT.

Acute and chronic graft-versus-host disease (GVHD) were assessed as described previously.6,26 Toxicities occurring within the first 100 days were scored using the Common Terminology Criteria for Adverse Events v3.0. Disease response after HCT was monitored with standard marrow morphology, flow cytometry and conventional cytogenetics, FISH and, if indicated, PCR for BCR-ABL transcripts.27 Disease responses were assessed 1, 3, 6, 12 and 24 months after HCT and/or as clinically indicated.

Patients with Philadelphia chromosome-positive acute lymphoblastic leukemia

Twenty-five patients had the Ph cytogenetic abnormality detected at diagnosis. After the introduction of imatinib, 18 patients enrolled in a study evaluating the safety and efficacy of incorporating post-grafting imatinib into the treatment regimen. Patients were initiated on imatinib at a dose of 600 mg orally once daily either by their referring physician or at the transplant center before enrollment on the study. Imatinib was stopped on day −2 before HCT to avoid interaction with engraftment of donor hematopoietic cells. Imatinib was recommenced at a dose of 400–600 mg daily after HCT when the absolute neutrophil count was greater than 0.5×10/L or on day +15 if there was no neutropenia. Imatinib was continued for at least 1 year after HCT, unless there was toxicity or disease progression; all patients received imatibib for at least 1 month. Dose reduction of imatinib was allowed for mitigation of side effects/toxicities.

Causes of death

In patients who relapsed or progressed with ALL, relapse/progression was listed as the primary cause of death regardless of other associated events. Relapse was defined as recurrence of malignancy based on one or more of the following parameters: marrow morphology, flow cytometry, cytogenetic studies, including FISH, or RT-PCR for BCR/ABL transcripts. All deaths occurring in the absence of relapse/progression were considered non-relapse mortality.

Statistical analysis

Data were analyzed as of October 1, 2010. Overall survival was estimated using the Kaplan-Meier method. Cumulative incidence estimates were calculated for acute and chronic GVHD, relapse and non-relapse mortality. Death was treated as a competing risk in the analyses of relapse/progression and acute and chronic GVHD. Relapse/progression was treated as a competing risk when analyzing non-relapse mortality. Cox regression was used for univariate analyses of risk factors for all time-to-event end points. For each analysis, hazard ratios (HR) and 95% confidence intervals (95% CI) are given together with P values for comparisons with the reference category. All P values are derived from likelihood ratio statistics and are two-sided.

Results

Fifty-one patients underwent allogeneic HCT. All had high-risk ALL including 19 who were beyond first complete remission. Six patients were under 18 years of age (1 in first complete remission and 5 beyond first complete remission). Table 1 summarizes the patients’ characteristics. Twenty-five patients had Ph+ ALL. Of these, 18 received post-grafting imatinib (Table 2). The median follow-up for surviving patients was 43 (range, 14–98) months.

Engraftment

Fifty patients achieved sustained donor engraftment. One patient with Ph+ ALL had non-fatal primary graft rejection. This patient received an HLA matched unrelated marrow graft with a dose of 0.9×10/kg CD34 cells. After graft rejection, this patient was treated with imatinib, chose not to undergo a second HCT, relapsed and died 20 months after HCT. The median donor CD3 T-cell chimerism levels for the 50 peripheral blood stem cells recipients at day 28 and day 84 were 79 (range, 15–100)% and 88 (range, 48–100)%, respectively. The chimerism and engraftment patterns were not different in patients treated with imatinib.

Graft-versus-host disease

By day 120 after HCT, 53% of patients had developed grade II acute GVHD and 6% had developed grades III-IV acute GVHD (Figure 1A). Among the patients who received HLA-identical sibling, HLA-allele-matched unrelated donor, and HLA-allele/antigen mismatched unrelated grafts, the overall incidences of grade II-IV acute GVHD were 33%, 50% and 91%, respectively. The cumulative incidence of chronic extensive GVHD at 3 years was 42% (Figure 1B).

Among the patients with Ph+ ALL treated with post-grafting imatinib (n=18), ten (56%) developed grade II-IV acute GVHD. There was no significant difference between the incidences of acute GVHD among patients who received or did not receive imatinib (HR=0.65, 95% CI 0.3–1.4; P=0.25). Ten of the 18 patients (56%) developed chronic extensive GVHD and five (28%) developed limited chronic GVHD. There was no significant difference in the incidence of chronic GVHD among patients who received or did not receive imatinib (HR=1.4, 95% CI 0.6–3.3; P=0.45). Five patients (36%) developed chronic skin GVHD while receiving imatinib treatment. Discontinuation of imatinib after HCT was not associated with new onset or exacerbation of chronic GVHD.

Table 1.Characteristics of ALL patients, disease and transplantation.

Non-relapse mortality

The rate of non-relapse mortality at 3 years after HCT among all patients was 28% (Figure 1C). The following causes of death were included under non-relapse mortality: GVHD (n=4), GVHD-associated infections (n=8), sepsis (n=1), congestive heart failure (n=1) and suicide (n=1). There was no significant difference in non-relapse mortality between patients who were or were not treated with post-HCT imatinib (HR=0.5, 95% CI 0.2–1.5; P=0.20). The four deaths in the imatinib group were due to GVHD-associated infections: bacterial pneumonia, sepsis with pancreatitis, respiratory syncytial virus pneumonia and community-acquired H1N1 viral pneumonia.

Disease relapse/progression

Among the 51 patients, 22 (43%) had relapse/progression at a median of 5 (range, 0.3 to 58) months after HCT. None of the patients developed isolated central nervous system relapse. None was treated with donor lymphocyte infusion for disease relapse. The median time from the diagnosis of relapse to death was 4 (range, 0.3–15) months in the 20 patients who had died by the time of analysis.

Table 2.Characteristics of Ph+ ALL patients treated with imatinib after HCT.

Overall, the 3-year estimated probability of relapse/progression was 40%. Univariate analysis identified risk factors for relapse/progression (Table 3). Patients beyond first complete remission at the time of HCT had a significantly increased risk for relapse after HCT compared to those in first complete remission (HR=3.9, 95% CI 1.6–9.5, P=0.002). For Ph– ALL (n=26), the 3-year estimated relapse rate for patients in or beyond first complete remission was 15% and 62%, respectively (Figure 2). For Ph+ ALL (n=25), the 3-year estimated relapse rate for patients in and beyond first complete remission was 32% and 67%, respectively (Figure 2). Two patients with molecular evidence of disease after transplantation were included as having disease relapse. For patients with Ph+ ALL, evidence of additional cytogenetic abnormalities at diagnosis was associated, at a borderline level, with an increased risk of relapse after HCT (HR=3.4, 95% CI 0.9–13; P=0.06).

Figure 1.Cumulative incidences (n=51) of (A) acute graft-versus-host disease (GVHD): 53% grade II, 6% grade III–IV; (B) chronic extensive GVHD: 42% incidence at 3 years; (C) non-relapse mortality, 28% incidence at 3 years.

Survival

The estimated 3-year overall survival rate among the 51 patients was 34%. Relapse was the primary cause of death (n=20, 57% of all deaths). Univariate analysis performed for the entire cohort identified the disease status of being beyond first complete remission as the only significant factor associated with increased mortality (HR=2.7, 95% CI 1.4–5.3; P=0.005) (Table 3). Other factors (donor source, acute or chronic GVHD) were not significantly associated with increased mortality. The 3-year overall survival rates for patients with Ph– ALL in and beyond first complete remission were 52% and 8%, respectively, (HR=3.4, 95% CI 1.3–9.1; P=0.01) (Figure 3A). Excluding the six pediatric patients, the 3-year overall survival rates for patients in and beyond first complete remission were 48% and 0%, respectively (HR=4.5, 95% CI 1.4–15.1, P=0.01).

Table 3.Prognostic factors for relapse and mortality using univariate analysis.

Among patients with Ph ALL, treatment with post-HCT imatinib was associated with significantly decreased mortality (HR=0.3, 95% CI 0.1–0.9; P=0.03) (Table 3). The 3-year overall survival rates for patients with Ph ALL who were in first complete remission or beyond first complete remission were 47% and 17%, respectively; however, this difference did not reach statistical significance (HR=1.8, 95% CI 0.6–5.4; P=0.32). For patients in first complete remission who received post-grafting imatinib, the 3-year overall survival rate was 62% (Figure 3B, Table 2); for the subgroup that had no evidence of minimal residual disease at HCT, the overall survival rate was 73%. Additional cytogenetic abnormalities (other than Ph) detected at diagnosis, showed a trend to be associated with increased mortality (HR=2.0, 95% CI 0.7–5.5; P=0.19).

Outcomes of patients under 18 years old

Six patients with Ph– ALL were younger than 18 years at the time of HCT: these patients had a median age of 11 (range, 8–16) years. Four had relapsed after a prior allograft, one was in first complete remission and one in third complete remission. Two patients received HLA-antigen mismatched unrelated grafts. All five patients beyond first complete remission relapsed (3 of them within 6 months). All patients developed acute GVHD. The single patient in first complete remission is currently alive at 88 months post-HCT.

Outcomes of patients aged over 60 years

Sixteen patients (31%) were older than 60 years at the time of HCT: the median age of these patients was 63 (range, 61–69) years. Six of the 16 (38%) patients died from causes other than relapse and four (25%) had disease relapse. Of the nine patients with Ph– ALL, two in first complete remission are currently alive after more than 2.1 years. Among patients with Ph+ ALL (n=7), the estimated 3-year overall survival rate was 57%.

Imatinib toxicity

The median duration of post-HCT imatinib treatment was 11.5 (range, 3–50) months (Table 2). The drug was given at a daily dose ranging between 200 mg and 600 mg. Dose modifications were made in three patients. In general, imatinib was well tolerated. Three patients (17%) discontinued imatinib because of adverse events, all of which were reversible (two cases of gastrointestinal toxicity and one of recurrent pleural effusion).

Discussion

Despite improvements in therapy, mortality from high-risk ALL has not decreased substantially in older patients. Two large prospective trials and meta-analyses summarizing the results of the previous controlled trials showed that allogeneic HCT after myeloablative conditioning improved the outcome of adult patients with high-risk ALL.2,28–31 However, in patients with high-risk disease, a survival advantage was demonstrated only up to 35 years of age.2,32 Developing HCT approaches for older or medically infirm patients with ALL has remained challenging. While the outcome for patients not undergoing allogeneic HCT is very poor, those who proceed with myeloablative conditioning followed by HCT have an unacceptably high rate of non-relapse mortality.2,33 This multicenter study addressed the problem of non-relapse mortality in older and medically infirm patients by using a non-myeloablative conditioning regimen consisting of fludarabine and 2 Gy total body irradiation which depended on allogeneic graft-versus-leukemia effects for curing high-risk ALL.

Figure 2.Cumulative relapse rate for Ph– ALL, in first complete remission (CR1) (n=13) versus beyond CR1 (n=13) and Ph⁺ ALL CR1 (n=19) versus beyond CR1 (n=6). Molecular disease relapse (PCR or flow cytometry positive) without morphological evidence of disease was included as relapse.

We observed that overall survival was significantly improved for patients who underwent HCT early in the course of their disease. For Ph– ALL patients in first complete remission, the 3-year overall survival rate was 52%, while that for patients beyond first complete remission was only 8%, primarily due to increased disease relapse. Other investigators who used various reduced intensity regimens and allogeneic HCT have reported 2 to 3-year overall survival rates ranging from 20%-61%.3,7–9,34 The heterogeneous study outcomes might have been due to differences in disease status and characteristics of the HCT donors. The positive impact of early allogeneic HCT in first complete remission was reported by Mohty et al. and Bachanova et al.3,7 Our study confirms both this finding and the poor overall survival of patients beyond first complete remission. Furthermore, the sustained survival plateau among patients in first complete remission demonstrates that durable, long-term disease-free survival may be achieved for a majority of Ph+ ALL patients treated with imatinib as well as for Ph– ALL patients. Thus, in patients for whom allogeneic HCT is considered as post-remission therapy, it should be recommended early rather than late in the course of the disease.

The most striking finding of our study was the favorable overall survival rate of 47% at 3 years for Ph+ patients in first complete remission given imatinib after HCT. The overall survival rate was 73% for those Ph+ patients in first complete remission without minimal residual disease at HCT. Relapse was increased in Ph+ patients: (i) with additional cytogenetic abnormalities, (ii) beyond first complete remission, and (iii) in first complete remission with minimal residual disease at HCT. The latter two factors were not statistically significant probably because of the inclusion of patients before the availability of imatinib. Age did not appear to limit the feasibility of our treatment protocol. For example, Ph+ ALL patients older than 60 years had a 3-year overall survival rate of 57%.

Figure 3.Overall survival for (A) Ph^– ALL, in first complete remission (CR1) (n=13) versus beyond CR1 (n=13) and (B) Ph⁺ ALL patients receiving imatinib after hematopoietic cell transplantation, CR1 (n=13) versus beyond CR1 (n=5).

Incorporation of imatinib and newer tyrosine kinase inhibitors into the various phases of the treatment for Ph+ ALL patients has reshaped the therapeutic algorithm. Thomas et al. showed that the incorporation of imatinib into induction chemotherapy resulted in a 2-year overall survival of 56%, which included patients who proceeded to myeloablative allogeneic HCT.35,36 A single center study showed that incorporation of imatinib into induction chemotherapy with subsequent myeloablative conditioning and allogeneic HCT resulted in a 3-year overall survival rate of 78%.37 Currently, data for patients treated with tyrosine kinase inhibitors and not proceeding to allogeneic HCT are scarce. A recent phase II trial showed a 2-year estimated overall survival rate of 64% for patients receiving only dasatinib in addition to induction and maintenance chemotherapy.38 While these results are encouraging, a median follow-up of 14 months in this cohort is probably too short to enable recommendation of alternative approaches to allogeneic HCT. Thus, it is possible that the major role of tyrosine kinase inhibitors, both in medically “fit” and in medically infirm or older patients, is to allow a greater proportion of patients to receive allogeneic HCT. Consequently, particularly in the setting of non-myeloablative HCT, the use of tyrosine kinase inhibitors post-transplant appears to provide a sufficient level of disease control until the development of a graft-versus-leukemia effect.

In accordance with published data for imatinib after myeloablative HCT,14 we showed that imatinib was safe in the context of non-myeloablative allogeneic HCT and was generally well tolerated. In contrast to recent publications that suggested a role for imatinib in the prevention and treatment of chronic skin GVHD,39,40 we did not observe a significant decrease in the incidence of chronic GVHD, nor did we detect an increase in the incidence of chronic GVHD after the discontinuation of imatinib, although the number of patients in our study was limited. The optimal duration of post-HCT imatinib therapy is unknown. In our study, patients were treated with imatinib for varying periods, according to preferences of the physicians and patients. Two patients were successfully treated with a second-generation tyrosine kinase inhibitor after detection of molecular relapse. Although the follow-up for these two patients was limited, the findings suggest that close follow-up of patients is necessary and that molecular complete remission can be achieved after detection of molecular relapse. Until data from larger cohorts are available, we would cautiously recommend continuing treatment with tyrosine kinase inhibitors indefinitely unless there is evidence of either toxicity or disease relapse.

Although the incidence of acute GVHD was high, most cases were grade II and only 6% of patients developed grades III-IV acute GVHD. In accordance with previous publications, the highest incidence of acute GVHD occurred in patients receiving HLA-mismatched grafts.24 In contrast to a previous study, we did not observe an inverse correlation between chronic GVHD and relapse.41 In part, this could be explained by the fact that relapse in our cohort occurred early after HCT (at a median of 5 months) particularly among patients with disease beyond first complete remission.

Since ALL is relatively rare in adults, our study was limited by the relatively small number of patients and the fact that enrollment occurred over a nearly 10-year period. Nonetheless, we showed that non-myeloablative conditioning and allogeneic HCT is a very feasible and effective treatment option for high-risk ALL patients in first complete remission. For Ph+ ALL patients in first complete remission, imatinib should be given after HCT since it appears to improve overall survival. Our results suggest that non-myeloablative conditioning and allogeneic HCT is a potentially curative treatment option for older or medically infirm patients with high-risk ALL in first complete remission. For patients beyond first complete remission, post-HCT maintenance with novel agents should be explored and studies addressing the role of next generation tyrosine kinase inhibitors given after HCT are warranted.

Acknowledgments

We thank the research nurses Michelle Bouvier and Hsien-Tzu Chen and data manager Gresford Thomas for their invaluable help in this study; Helen Crawford, Bonnie Larson, and Sue Carbonneau for manuscript preparation; and especially the patients and their families, the transplantation teams, physicians, nurses, long-term follow-up team and support personnel for their dedicated care of patients in this study.

Footnotes

• Funding: this work was supported by grants from the National Institutes of Health, Bethesda, MD (grants P01CA018029, P01CA078902, and P30CA015704). RR was a recipient of a fellowship award from the Davidoff Foundation.
• Authorship and Disclosures The information provided by the authors about contributions from persons listed as authors and in acknowledgments is available with the full text of this paper at www.haematologica.org.
• Financial and other disclosures provided by the authors using the ICMJE (www.icmje.org) Uniform Format for Disclosure of Competing Interests are also available at www.haematologica.org.

• Received January 7, 2011.
• Revision received March 29, 2011.
• Accepted April 11, 2011.